Question

Question 47.
A sulphate salt of group 2 element of the periodic table is a white, soft substance, which can be moulded into different shapes by making its dough. When this compound is left in the open for some time, it becomes a solid mass and cannot be used for moulding purposes.
Identify the sulphate salt and why does it show such a behaviour ?
Write down the reaction involved.

Answer 1

The sulphate salt which is used for making different shapes is plaster of Paris. Its chemical name is Calcium Sulphate hemihydrate $({\text{CaSO}}_4.\frac{1}{2}{\text{H}}_2\text{O})$. The two formula units of ${\text{CaSO}}_4$ share one molecule of water. As a result, it is soft. When it is left open for some time, it absorbs moisture from the atmosphere and forms gypsum which is a hard solid mass. So, gypsum sets as a hard solid mass and cannot be used for moulding purposes.

$\underset{\text{(Plaster of Paris) Sulphate salt(soft)}}{{\text{CaSO}}_4.\frac{1}{2}{\text{H}}_2\text{O}}+\underset{\text{Water}}{1\frac{1}{2}{\text{H}}_2\text{O}}⟶\underset{\text{Gypsum (hard mass sets as)}}{{\text{CaSO}}_4.2{\text{H}}_2\text{O}}$